Equilibrium climate sensitivity (ECS) is the warming that would occur after doubling CO₂ (from 280 to 560 ppm) and waiting for the system to reach equilibrium. IPCC AR6 estimates it at 2.5–4 °C (best estimate: 3 °C).
This uncertainty range stems mainly from feedbacks involving clouds, ice, and water vapour. A sensitivity of 3 °C implies that at current emission rates, we will reach +2 °C above pre-industrial around 2040–2050.